Dynamic nonlinear optical characterization of asymmetric pentaazadentate porphyrin-like metal complexes for optical power limiting application

We introduce a new class of two-dimensional conjugated ring nonlinear optical materials, the asymmetric pentaazadentate porphyrin-like metal complexes [(R-APPC)M]Cl_n, for optical power limiting (OPL) application. Series of [(R-APPC)M]Cl_n complexes with variability in the phenyl substituents, in the bridging aromatic group in the ring, and in the metal center are characterized for third-order nonlinear optical properties and dynamic photophysical properties.; Degenerate four-wave mixing of [(R-APPC)M]Cl_n shows large molecular second-order hyperpolarizabiIities, γ, even greater than those of metallophthalocyanines, a well known class of optical limiting materials. The effects on γ due to the structural changes are discussed along with the effect on the linear absorption spectra.; Optical limiting performances of the series are investigated using nonlinear transmission measurements at 532 nm with 5 ns pulses. The results exhibit low limiting threshold fluence and limiting throughput fluence comparable to silicon naphthalocyanine and lead phthalocyanine. The effects of structural variations on nonlinear absorption properties are discussed.; Because excited state absorption is the dominant nonlinear optical process for this series, the dynamic nonlinear properties are discussed from the results of time-resolved degenerate four-wave mixing (TRDFWM). It is found that at least two different dynamic processes are observable in TRDFWM at 532 nm using a mode-locked picosecond laser. One is a fast process between singlet excited states limited by excitation pulse width (∼40 ps), and another is a relatively slower process involving intersystem crossing to the triplet excited state from the singlet excited state.; To understand excited state mechanism further, the lifetime of the singlet excited state has been measured for each complex in the series using the time-correlated single photon counting (TCSPC) method with a femtosecond Ti:Sapphire laser and a fast microchannel plate photomultiplier. The TCSPC results show that the lifetimes of [(R-APPC)M]Cl_n are all less than 1 ns. From the previously reported triplet quantum yields, the intersystem crossing rates are found to be quite fast (<1 ns).; These results suggest that [(R-APPC)M]Cl_n complexes are very promising candidates for OPLs because these complexes have large optical nonlinearities and exhibit higher flexibility in structures.